[Frontend] Fix bad merge causing test failures in Index/Store/handle-prebuilt-module.m
[Frontend] Fix bad merge causing test failures in Index/Store/handle-prebuilt-module.m

Bad merge in CompilerInstance.cpp in 142bd7968f4399c1f95e9bc75665c028889390dc.
Resolves rdar://problem/38880213

apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang